This invention relates to filter media used to remove contaminants from air or other gases. More particularly, the invention relates to a method for impregnating a filter media where by at least one impregnate is deposited by sublimation and at least one other by non-bulk absorption in order to provide broad spectrum filtering performance.
Extended surface area substrate particles, such as activated carbon, alumina, zeolites, and the like, are widely used in air filtration because of the ability of such materials to remove a wide range of different materials. The filtration characteristics of these materials arises from a highly porous or convoluted surface structure. In the case of activated carbon, the surface porosity results from controlled oxidation during the xe2x80x9cactivationxe2x80x9d stage of manufacture. Activated carbon has been used for air filtration for many decades.
The ability of the carbon to remove a contaminant from the air by direct adsorption depends on a molecular-scale interaction between a gaseous molecule and the carbon surface. The extent of this interaction depends upon factors that include the physical and chemical surface characteristics of the carbon, the molecular shape and size of the gaseous compound, the concentration of the gaseous compound in the gas stream to be filtered, residence time in the carbon bed, temperature, pressure, and the presence of other chemicals. As a rule of thumb, for a single contaminant, the extent of adsorption is primarily dependent on boiling point. In general, the higher the boiling point, the greater the capacity of carbon to remove the chemical.
Accordingly, carbon does not have a great capacity by itself to remove lower boiling point gases. Treatments have been devised in which chemicals are coated on the carbon to provide filtering capabilities towards lower boiling point gases. These treatments are generally known as xe2x80x9cimpregnationxe2x80x9d methods, and the result of treatment is an xe2x80x9cimpregnatedxe2x80x9d carbon.
Over the course of this century, development of impregnation techniques has progressed so that a variety of impregnants are available for removing a wide range of different chemicals. Progress has been accelerated during wartime, when actual and perceived threats spurred the development of specialized carbons. However, there has hitherto been a distinction between the types of filter media particles used for military applications, and those used in industrial applications. Military requirements have made it necessary for filter media particles to be capable of removing a range of chemicals, and so multi-component impregnation formulations have been devised. In industry, where the nature of hazards is known in advance, the practice has been to select a filter appropriate to the known hazard. Consequently, filters with capability toward a specific type of chemical or class of chemicals have developed for industrial applications.
Over time, regulatory structures for the selection and use of respiratory protective equipment have evolved, along with approvals systems to ensure that designs of equipment on the market are capable of meeting necessary performance requirements. Such approvals systems have been generated for industrial purposes across international boundaries. These include the European Norm system that is adopted widely in Europe and elsewhere in the world. Another example are the approvals requirements of the US National Institute for Occupational Safety and Health that have been adopted in the USA, Canada and certain other countries. For military requirements, performance specifications are determined by each national need, although there arc some internationally agreed upon standards under the North Atlantic Treaty Organisation.
The first U.S. patent for a treatment of carbon to remove a variety of military gases derived from developments to protect personnel in World War I battles in which chemical agents were used in excess. The patent by Joshua C. Whetzel and R. E. Wilson (U.S. Pat. No. 1,519,470, 1924) described the use of an ammoniacal solution of copper carbonate to impregnate a granular activated carbon. This technique became known as xe2x80x9cWhetlerizationxe2x80x9d, and the carbon product xe2x80x9cWhetleritexe2x80x9d. Variations on this technique have been developed over time. (U.S. Pat. No. 2,902,050, U.S. Pat. No. 2,902,051, DE 1,098,579, FR 1,605,363, JP 7384,984, CZ 149,995).
During World War II substantial technical investigations were made into the use of impregnated carbons. The U.S. research in this area is summarized in xe2x80x9cMilitary Problems with Aerosols and Nonpersistent Gasesxe2x80x9d, Chapter 4: xe2x80x9cImpregnation of Charcoalxe2x80x9d, by Grabenstetter, R. J., and Blacet, F. E., Division 10 Report of US National Defense Research Committee (1946) pp.40-87. This report provides in depth coverage of a number of impregnant formulations.
The United Kingdom pursued a slightly different impregnation approach. There, copper oxide was mixed with coal prior to carbonization and activation, so that the activated carbon contained metallic copper distributed throughout its structure. This material was the basis for the filter carbons used in World War II.
The ability of the carbon to remove cyanogen chloride (CK) was improved by the application of the amine pyridine or, separately, by impregnation with chromium in the form of sodium dichromate. This form of carbon, in combination with a pyridine impregnant, was used in military respirator filters manufactured in the 1970s.
Post World War II research has explored how the addition of organic compounds to impregnated carbon could improve the shelf life. Experiments were undertaken in the UK, France and elsewhere with various amines. One such material found to improve the shelf life towards cyanogen chloride is triethylenediamine (also known as TEDA or 1,4-diazabicyclo-[2.2.2]-octane). When impregnated on carbon, TEDA has been found in its own right to be capable of reacting directly with cyanogen chloride and is also highly capable of removing methyl bromide and methyl iodide. TEDA is strongly adsorbed onto carbon, is stable, is effective at low levels, and has minimal toxicity compared with other amine compounds. TEDA is a solid at room temperature, but sublimes readily.
Chromium has traditionally been used as a carbon impregnant in military applications, as it facilitates the satisfactory removal of hydrogen cyanide and cyanogen chloride (CK). Because the hexavalent ionic form of chromium has been identified as a potential lung carcinogen, work undertaken in recent times and dating back to the early 1970""s has explored formulations that avoid or reduce the level of chromate salts as impregnants.
In recent times, the traditional role of military forces has changed from a more or less predictable battlefield conflict to encompass peace-making and peace-keeping roles, and supporting civilian authorities in emergency response. Such activities may involve responding to the release of chemicals by accident or intent. Intentional release of chemicals, referred to as xe2x80x9cchemical terrorismxe2x80x9d, has occurred in fact and been threatened numerous times. These incidents may involve chemicals that have been traditionally regarded as military threats or may involve hazardous chemicals normally used in industry. The response to these hazards is ultimately likely to involve both civilian and military authorities and is likely to require protection systems that meet industrial approvals as well as military performance requirements.
Filtration-based protection systems are appropriate for personnel undertaking various tasks at some distance from a point of chemical release. For such cases, it is most desirable to be able to respond to a hazard quickly and without delay. Conventionally, however, delay may be inevitable as it may be necessary to first identify a threat in order to select an appropriate filter. In order to be able to respond to a wide range of possible hazards, it has been necessary to carry inventories of many different kinds of filters. It would be much more desirable to have one filter type that can provide protection against many different hazards. Such a multi-purpose filter desirably would accommodate both industrial and military needs.
U.S. Pat. No. 4,531,953 describes a method by which an amine such as TEDA is added to carbon via sublimation. The method must occur in the absence of added water. Otherwise, the patent indicates that the water would use up the adsorption capacity of the carbon, limiting the amount of amine that could be adsorbed. Unfortunately, this approach provides impregnated carbons with reduced moisture content. Because water can help filter acid gases, this impairs protection against acid gases.
Thus, contrary to the teachings of U.S. Pat. No. 4,531,953, it is often desirable to include or enhance the moisture content of filter media particles for acid gas protection. However, adding or maintaining moisture causes problems. As noted in U.S. Pat. No. 4,531,953, water could occupy valuable absorption capacity of the particles.
Additionally, because many effective impregnants (e.g., amines, metal salts, etc.) are water soluble, conventional methods for adding water can wash away impregnants from filter media particles.
Clearly there is a need for incorporating both solid organic impregnants (such as TEDA) and a fluid impregnant (such as water) onto carbons without unduly compromising filtering performance.
The present invention provides an approach for beneficially incorporating both a solid organic impregnant and a fluid impregnant into filter media particles without the presence or addition of one unduly affecting the performance of the other. The present invention achieves impregnation by combining sublimation and non-bulk absorption techniques. These techniques may be carried out in sequential, simultaneous, overlapping, and/or alternating fashion, or the like. The invention is especially suitable when the filter media particles already contain or are to contain one or more other impregnants, whose function or presence might be adversely affected when conventional methods are used to add the solid organic and fluid materials. For example, if any of the impregnants are water soluble materials (such as some amines and most metal salts), trying to add water to the particles via immersive contact with an aqueous liquid can wash the water soluble materials away. The present invention substantially avoids this problem.
Accordingly, it can be appreciated that the present invention is particularly useful when it is desired to increase the water content of filter media particles to achieve enhanced acid gas protection. The present invention is advantageously practiced in connection with filter media whose performance is enhanced by multiple impregnants. In particularly preferred embodiments, the present invention may be advantageously used to incorporate both a solid amine such as TEDA and a solvent such as water onto filter media particles.
As an overview of a preferred methodology, the filter media particles to be impregnated are placed into a suitable processing vessel along with the desired amount of solid, organic compound(s). Processing is carried out under conditions effective to allow the solid organic compound to sublime and impregnate the filter media particles. During at least one or more portions of the sublimation, the desired fluid impregnant is introduced into the vessel so as to prevent bulk absorption by the filter media particles so that at least one or more portions of the sublimation is carried out in the presence of the fluid. Preferably, the fluid is introduced as a vapor (steam in the case of water) or as atomized droplets, streams, mist or fog via suitable nozzle structure(s) to prevent bulk absorption conditions. Advantageously, this methodology allows both kinds of materials to be incorporated into filter media particles without the addition or presence of one of the materials unduly compromising the performance of the other. As an additional advantage, preferred embodiments of the invention produce substantially no waste as all input materials are quantitatively included in the finished composition.
In contrast to the practice of the present invention, xe2x80x9cbulk absorptionxe2x80x9d or xe2x80x9cimmersive contactxe2x80x9d of a fluid impregnant refers to contact in which the particles to be impregnated are caused to directly contact a liquid bath comprising the fluid impregnant. In a preferred sense, bulk absorption by a porous solid material is characterized by the penetration of a liquid into a solids porous matrix under conditions in which the outer surface(s) of the solid are in communication with a large reservoir of liquid that has a volume in excess to the air displaced from the solid during absorption. Bulk absorption can occur via penetration absorption and/or immersional absorption mechanisms. With penetration absorption, bulk liquid penetrates a solid matrix from one side and displaces air from the solid through the open spaces in the matrix as the liquid front moves through the matrix. In a penetration absorption mechanism, free liquid is present even after the fill saturation of the solid. Immersional absorption occurs when a porous mass is totally immersed in a bulk liquid and the liquid penetrates into the solid generally from all directions.
In the practice of the present invention, xe2x80x9cnon-bulk contactxe2x80x9d or xe2x80x9cnon-immersive contactxe2x80x9d means that the fluid is caused to impregnatingly contact the particles in a form other than via bulk absorptionIn a preferred sense, the absorption impregnation conditions of the invention would include mechanisms such as capillary condensation and liquid thread motion. In capillary condensation, vapor is condensed into the pores or crevices within a porous solid. Absorption by liquid thread motion would occur when a droplet of liquid contacts the outer surface of the solid, wets its surface an is drawn into the porous volume.
Examples of non-bulk contact, or non-imnmersive contact, include causing the fluid to contact the particles as a vapor, gas, one or more streams, droplets, mist, fog, combinations of these, or the like. In preferred embodiments, the amount of fluid caused to contact the particles in a non-bulk contact is substantially equal to the amount of the fluid that is desired to be absorbed onto the particles. Advantageously, this significantly reduces, or even eliminates, any waste associated with the impregnation process. In this context, xe2x80x9csubstantially equalxe2x80x9d means that the amount of fluid is within 20%, preferably 10%, more preferably 1% of the desired absorption amount. For example, if 100 parts by weight of particles are to be impregnated with 2% by weight of the fluid, then generally about 1.5 to 2.5 parts by weight of the fluid would be caused to contact the particles via non-bulk contact.
In one aspect, the present invention relates to a method of making a filter medium. At least one solid, organic compound is caused to sublime onto the extended surface area of a plurality of filter media particles. During at least a portion of the sublimation, a fluid impregnant is caused to be absorbed by the filter media particles via non-bulk contact
In another aspect, the present invention relates to a method of making a filter medium. A plurality of filter media particles is intermixed with a plurality of solid amine particles to form a solid mixture. The solid mixture is heated under conditions effective to cause at least a portion of the amine to sublime onto the filter media particles. While heating the solid mixture, the particles are non-immersively contacted with a fluid impregnant to achieve non-bulk absorption of at least a portion of the fluid by the particles.